There is currently much technical interest in a wide range of devices in which radiation at wave lengths of 0.3 .mu.m to 4 .mu.m is generated in the core of an optical fibre. In these devices the fibre contains a fluorescent additive which interacts with excitation radiation, usually identified as the "pump radiation", to produce the desired output. The devices take many forms, e.g. broadband sources, super luminescent sources and temperature sensors, but devices which display laser activity are particularly important, especially in telecommunications. It should be realised that telecommunications uses laser activity in two distinct manners namely optical oscillators and optical amplifiers. However the same doped glass fibres are equally suitable for a plurality of such applications (and often for all such applications).
Stone and Nurrus in "Applied Physics Letters" (Volume 23, No. 7, 1 October 1973 at pages 388 and 389) disclose lasers made of neodymium doped silica with end-pumped fibre geometry. One of their systems has an active core of fused SiO.sub.2 and Nd.sub.2 O.sub.3 enclosed in a thin passive sleeve of SiO.sub.2 and Nb.sub.2 O.sub.5 all enclosed in a fused jacket of SiO.sub.2. The diameter of the active core was about 800 to 15 .mu.m and the length of the samples was 1 cm. The function of the thin passive sleeve is to increase the guidance of the core and hence pump efficiency.
U.S. Pat. No. 3 808 549 describes an optical waveguide light source having an active core surrounded by an inner cladding layer and an outer cladding layer. The refractive index of the outer cladding is lower than the refractive index of the inner cladding which is lower than the refractive index of the core. Pump radiation is launched into the inner cladding layer to which it is confined by the outer cladding. The pump radiation makes many passes through the core whereby its absorption by the core is increased. The signal is generated within the core.
It has long been recognised that the rare earth elements, e.g. Nd, Er, and Yb, display fluorescent properties which make them suitable for use as fluorescent additives in optical fibre. Their fluorescent properties make them particularly suitable for use in the laser devices mentioned above. The operation of a fluorescent device clearly depends on absorption of the pump photons in order to raise ions (or other fundamental particles) of dopant to an excited state for subsequent emission on a different transition. In a laser device, this emission is stimulated by the presence of a signal photon and, therefore, the operation of a laser device also depends on the interaction of radiation at signal wave length. It is an object of this invention to make efficient use of pump power launched into optical fibre. In the case of optical amplifiers this means achieving high gain for small launched pump powers whereas for optical oscillators it implies a low lasing threshold.
Fibre according the invention has a fluorescent additive unevenly distributed over the cross section of the core and having a higher concentration of the additive at the centre of the core than at the core/cladding boundary. The highest concentrations of additive should ideally be located in those regions of the fibre where, during pumping, the highest intensity of pump radiation is to be expected. Lower or zero concentrations of the additive should be located where only low pump intensities are to be expected.
In most pumping schemes the highest intensity of the pump radiation will be located at the centre of the core and it is appropriate to provide the highest dopant concentration at the centre of the core with zero concentration at its periphery. Preferably the core comprises two zones, namely an inner zone surrounded by an outer zone wherein the inner zone contains the dopant and the outer zone contains substantially no dopant. Suitably the inner zone constitutes less than a quarter, e.g. 5 to 15%, of the cross sectional area of the core.
The fibre may be implemented in any glass system which is compatible with the fluorescent dopants. Thus, for example, the fibre may be implemented in conventional silicate, phosphate and fluoride systems, eg. fluorides of Zr, Ba, La, Al, Na and Hf or in silica systems, eg. SiO.sub.2 with additives such as GeO.sub.2 to adjust the refractive index in the core.
In a specific embodiment silica fibre has:
(a) a cladding formed of SiO.sub.2 with P.sub.2 O.sub.5 to reduce the melting point, and F to offset the increase in refractive index, PA1 (b) an outer core region formed of SiO.sub.2 with GeO.sub.2 to increased the refractive index and P.sub.2 O.sub.5 to reduce the melting point; and PA1 (c) an inner core region formed of SiO.sub.2 with Al.sub.2 O.sub.3 to increase the refractive index, P.sub.2 O.sub.5 to decrease the melting point and prevent devitrification and a fluorescent dopant to interact with the pump radiation. PA1 (1) The exposed surface is so small that the rate of loss is minimal. PA1 (2) The final stage only takes a time which is too brief for noticeable loss to occur.
The dimensions of the fibre are preferably such that it is single mode at signal wave length. This implies that it may be able to support several, e.g. up to 4 or 5, modes at pump frequency. The fluorescent dopants of major interest include the rare earth metals. Of these the most important are Er (which lases by the three level mechanism) and Nd (which lases by the 3 and four level mechanism)
One method of making silica fibre according to the invention utilises the modified chemical vapour deposition process usually identified as MCVD. MCVD is sometimes known as the inside deposition process because the glasses which eventually form the operative parts of the fibre are produced by converting the equivalent chlorides into the desired oxides which are deposited, layer by layer, on the inner surface of a substrate tube. Usually a total of 10 to 30 layers are deposited. As initially deposited the glass is porous but the porous material is immediately fused to give a solid layer upon which subsequent layers are deposited. When all the layers have been deposited the tube is collapsed to a rod which is drawn into fibre.
To make fibre according to the invention this procedure is followed for the cladding and the outer core. The precursor of the inner core is deposited but left in the porous state. Dopants, including the fluorescent additive, are introduced as solution into the porous layer. After solvent removal and conversion to oxides as necessary, the porous layer is consolidated and the tubular configuration is collaped into a rod which is then drawn into fibre.
It will be appreciated that this, i.e. soaking a solution into a porous layer, is one of many known techniques of introducing dopants into optical fibre. It has been adapted, in accordance with the invention, to provide a small, doped centre region in a larger core. One difficulty inherent in MCVD is that there is a tendency to lose dopant by evaporation from the exposed inner surface. This is not acceptable since the invention requires a high concentration of dopant at the axis. The depleted zone can be removed, e.g. by etching, just before final collapse. Although there appears to be a risk that further loses could occur during the final stage of the collapse, this does not happen to any noticeable extent because:
However we have most surprisingly discovered that, when aluminium is used to adjust the refactive index of the core, the loses of fluorescent dopant are not noticeable. The aluminium can be introduced at the same time as the fluorescent dopant, e.g. as Al(NO.sub.3).sub.3 in alcoholic solution. During heating the Al(NO.sub.3).sub.3 is converted to Al.sub.2 O.sub.3.
The fibre according to the invention can be used to make conventional fibre devices which include a pump for providing excitation radiation for the fluorescent additive.